In several types of papermaking processes, water soluble wet strength resins are added to the pulp furnish to increase the end use wet strength properties of the paper products. Items such as paper towels, napkins and tissues as well as other specialty paper grades are formed from pulp furnish which includes wet strength resins. These wet strength resins enhance the strength of the end product when wet.
Wet strength resins which allow the end product paper to retain more than 15% and up to 50% of its dry strength when wet are well known in the papermaking art. Various types of wet strength resins include ureaformaldehyde, melamine-formaldehyde, polyacrylamide, poly(aminoamide)-epichlorohydrin resins and their complex derivatives which are employed as pulp furnish components. The poly(aminoamide)-epichlorohydrin (PAE) type resins have become the most commercially important thermosetting resins and dominate the current paper wet strength resin market. PAE resins are water soluble cationic polymers which are typically added to the pulp furnish at an intermediate degree of polymerization so that the final cure of the polymer occurs in the dryer section of the paper machine. PAE resins are used extensively because they are neutral to alkaline curing, they impart permanent wet strength properties and they provide long shelf life. Also, because PAE resins are water soluble cationic polymers, they are effectively retained on anionic cellulosic fibers. By water soluble, it is meant that the resins are water soluble at the time they are added to the papermaking furnish. Subsequent events such as crosslinking can render the resins insoluble in water. Wet strength resins are generally believed to undergo crosslinking or other curing reaction after they have been deposited on, within, or among the papermaking fibers.
The manufacture of paper typically involves the processing of a carefully prepared aqueous fiber suspension (the pulp furnish) containing chemical additives to produce a highly uniform dry paper. Three steps included in the typical paper process are sheet forming where the suspension is directed over a porous synthetic mesh or "wire"; sheet pressing, where a formed sheet is passed through presses covered with belt-like porous felts to extract retained water from the sheet and to transfer the delicate sheet to the next final step of paper drying, commonly known as "yankee drying" in the case of tissue and towel grade papers.
Press felts commonly circulate continuously in a belt-like fashion between a sheet contact stage and a return stage. During the sheet contact stage, water along with other contaminants and additives is drawn from the sheet, usually with the aid of press rolls and/or a vacuum, into the pores of the felt and then subsequently removed from the felt.
The quality of the aqueous fiber suspension used to produce the sheet is dependent upon many factors including the composition of any recycled fibers added to to the process as well as the additives used in the preparation of the paper furnish. Thus, a variety of dissolved or suspended materials can be introduced into the manufacturing process, including both organic and inorganic materials such as talc, rosin, pitch, lignin, wet strength resins, cationic or anionic retention aids, water treatment chemicals, fines, anionic trash resins, calcium carbonate, clay, kaolin, silicon dioxide, titanium dioxide, alum, hydrolized AKD and ASA size, starch coating from broke, binding resins, ink particles, toners, dyes, etc. The ultimate result of these well intended additives in the aqueous fiber suspension is that they tend to deposit in the fine pores of the porous felts if they don't become an integral part of the sheet and are not removed from the felt continuously by chemical and/or mechanical means. The presence of wet strength resins in the process system compounds the problem. When PAE resin is present in the system, it tends to crosslink with itself and size the press felt fibers and render the felt surface nonabsorbent over time. In this situation, serious machine runnability and quality problems occur, ultimately leading to significant production loss. This is particularly true in current operations where the press felts are made of a polyamide fiber (nylon with various variations in fiber size, base structure, density, porosity, surface treatment, etc.) which generally carries a negative surface charge and possesses a high affinity for PAE type wet strength resins.
To control these problems related to PAE contaminated press felts, the felts have traditionally been mostly batch washed since few continuous felt conditioning chemicals are known to be effective. Batch cleaning products typically contain alkali, chelants, surfactant compositions and some solvents or in more severe cases, sodium hypochlorite solution is used alone. Even with these harsh and frequent batch wash treatments, PAE type wet strength resins along with other contaminants have been found difficult to remove. The use of sodium hypochlorite is relatively more effective for removing PAE resin but its detrimental effect on polyamide (nylon) fibers causes other problems such as loss of batt fibers from the felt (fiber shedding) and weakening of the felt's integrity, etc. leading to premature felt damage and short felt life. In addition, the use of chlorine based reagents is being limited in the paper industry due to increased environmental and regulatory restrictions. Batch cleaning operations where the machine is shut down also result in significant production losses.
Continuous felt conditioning chemical treatments based upon various nonionic or anionic surfactants, solvents, dispersants, etc. have been employed in the art. However, current continuous felt conditioning treatments have shown very limited efficacy toward controlling the PAE wet strength resin deposition in press felts.